Dim stars tell the age of the universe

White dwarfs indicate that cosmos is 13 billion to 14 billion years old

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April 24, 2002 — The Hubble Space Telescope has read the embers of burnt-out stars to come up with a new way of estimating the universe’s age, scientists said Wednesday. The latest age estimate — 13 billion to 14 billion years — is consistent with the conclusions reached using other methods.

Past observations of faraway celestial objects known as Cepheid variable stars have yielded estimates of 12 billion to 15 billion years for the universe’s age. But those observations depended on assumptions related to how the universe has expanded over those billions of years.

Another method, used by the European Southern Observatory, depended on estimating the rate of radioactive decay within old stars. The European researchers came up with a figure of more than 12.5 billion years.

University of Chicago astrophysicist Michael Turner, commenting on the latest research, said that further studies were essential because the question of the universe’s age is so fundamental to our understanding of the cosmos.

“What’s exciting is, we’re now making enough measurements that we can do these critical tests for age consistency,” he told MSNBC.com.

The latest test involves measurements of ancient white dwarfs, dim stars whose nuclear furnaces have gone out long ago. Astronomers say such stars cool down at a predictable rate: The older the dwarf, the cooler it is. That means the temperature of a white dwarf can be used as a “clock” to figure out how long it’s been cooling.

Astronomers compare the feat to figuring out how long ago a campfire was burning by measuring the temperature of the smoldering coals.

“This new observation short-circuits getting to the age question, and offers a completely different way of pinning down that value,” said the Hubble team’s principal researcher, Harvey Richer of the University of British Columbia. His team’s results, announced Wednesday at a NASA briefing, are to be published in the Astrophysical Journal Letters.

Spotting the dwarfs
The tricky part was finding the oldest, dimmest white dwarfs. The Hubble astronomers looked within M4, a globular star cluster located 7,000 light-years away in the constellation Scorpius. M4 was chosen because such globular clusters are thought to be the oldest features of our Milky Way galaxy, and because M4 is the closest globular cluster — which would make it easier to spot extremely faint objects.

The white dwarfs found in M4 are less than a billionth the apparent brightness of the faintest stars that can be seen by the naked eye. That faintness is well beyond the specifications of Hubble’s original cameras — but they were detected nonetheless, after eight days’ worth of exposures over a 67-day period. More than 600 white dwarfs were analyzed, Richer said.

“If there were any white dwarf stars fainter than the faintest objects that he sees, they would have appeared,” Bruce Margon, associate director for science at the Space Telescope Science Institute, said at the NASA briefing in Washington. “But they don’t.”

The scientists determined the temperature of the faint stars, and calculated that the stars were just less than 13 billion years old, Richer said.

“That’s the age of the star cluster,” Richer said. “We don’t believe that the star cluster formed instantaneously after the Big Bang.”

Other Hubble observations have indicated that the first stars formed less than 1 billion years after the birth of the universe. Adding those years onto the white dwarf figure yields the ballpark age estimate of between 13 billion and 14 billion years.

“This is state of the art,” said Wendy Freedman, an astronomer at the Carnegie Observatories who participated in the previous round of “age-of-the-universe” research. Her calculations, adjusted to account for the accelerating expansion of the universe, came up with the same ballpark estimate.

The cosmic picture
Astronomers acknowledged that there was still a billion-year uncertainty factor in their estimates. They said new instruments such as Hubble’s Advanced Camera for Surveys could provide further data for their calculations. However, they said it was questionable whether the age of the universe could be calculated with much more precision.

“Even to get this as a minimum age is incredibly valuable,” Margon said.

Richer joked that knowing the age of the universe probably isn’t “going to help you do well in the stock market.” But he and other astronomers pointed out that the question was fundamental to the various models of the universe’s birth, development and ultimate fate — and to our everyday conception of our place in cosmic history.

“We have on very firm footing that we live in an evolving universe that began 14 billion years ago, not 6,006 years ago,” Turner told MSNBC.com. “We have the beginning of all this under control. Now we want to know what the destiny is.”